Magnetic circuit comprising a ferromagnetic part having high permeability and a substantially flat, thin permanent magnet



Jal 4, 1955 A T. VAN URK ET AL 2,698,917

MAGNETIC CIRCUIT COMPRISING A FERPOMAGNETIC P ART HAVING HIGH PERMEABILITY AND A SUBSTANTIALLY FLAT, THIN PERMANENT MAGNET Filed Jan. 4, 1952 i2 INVE NTOR S Arend Thomas van Urk United States Patent O MAGNETIC CIRCUIT COMPRISING A FERROMAG- NETIC PART HAVING HIGH PERMEABILITY AND A SUBSTANTIALLY FLAT, THIN PERMA- NENT MAGNET Arend Thomas van Urk and Adriaan Rademakers, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application January 4, 1952, Serial No. 264,984 Claims priority, application Netherlands April 23, 1951 8 Claims. (Cl. 317-201) The invention relates to magnetic circuits comprising a ferromagnetic part having a high permeability and a substantially at, thin permanent magnet, magnetized substantially in the direction of its smallest dimension and made of a material having high coercive force BHC of at least 750 Oersted and a comparatively low remanence (for example, less than 5,000 Gauss), the smallest dimension being less than 1A of the largest outer dimension of one pole surface. The ferromagnetic part comprises a portion in which the path of the lines of force is parallel to the lines of force in the magnet and the magnetic circuit comprises atleast one airgap.

According to the invention, a magnetic circuit cornprising a ferromagnetic part having a high permeability and a substantially at, thin permanent magnet magnetised substantially in the direction of its smallest dimension, and made of a material having high coercive force, BHC of at least 750 Oersteds and a comparatively low remanence (of, for example, less than 5,000 Gauss), the smallest dimension being smaller than one quarter of the largest outer dimension of one of the pole surfaces, the ferromagnetic part comprising a portion in which the path of the lines of force is parallel to the lines of force in the magnet, whilst in the magnetic circuit at least one airgap is provided, is characterized in that the said portion is spaced apart from the magnet by a distance which is smaller than half, preferably smaller than one quarter the length of the magnet between the poles, the airgap being provided at least in the proximity of an edge of the magnet. Although there is no objection from a magnetic point of view to reducing the said distance to zero, practical considerations make it desirable for the said distance to be about l mm.

The invention is based on the discovery that certain permanent magnetic materials, more particularly those which have a high IHC-value with respect to the coercive force BHC and a permeability u of less than 2, for exam` ple, l to 1.6, are found to exhibit the property. The number of stray lines of force emanating from the sides between the poles is increased only slightly by arranging ferromagnetic material of high permeability, such as soft iron, on the sides of the magnet, which does occur, on the contrary, for example with the known magnetic steels on an Fe, Aland Ni-basis to which Co, Cu and/or Ti is added. Consequently, the invention is restricted to the use of permanent magnetic materials which exhibit the aforesaid properties.

Apart from the aforesaid property such materials have a further property in that, if desired, the magnet may be magnetised outside the circuit without the risk of real demagnetisation when the magnet is transferred from the magnetising devise to the nal magnetic circuit without any precautions being taken. Consequently, magnetic keeping by means of a soft iron circuit provided between the two pole surfaces during the transfer of the magnet may be dispensed with. It is even found that substantially no change of the working point is produced, if the magnet is taken out of the circuit and reintroduced into it without any precautions being taken. This, consequently, means that the so-called reversible curves substantially coincide with the demagnetisation curve or BH-curve. It is found that, even in the presence of very strong external, opposite magnetic fields, demagnetisation of the material is eifected in practice with great diiiiculty only.

Patented Jan. 4, 1955 Finally, some of these materials have a comparatively high electrical resistance.

The use of the invention has the advantage that, owing to the compact construction obtainable the size of the magnetic circuit may be comparatively small.

The magnetic circuit according to the invention is of particular importance, if the permanent magnet is made of a material described and claimed in U. S. application Ser. No. 239,264 tiled July 30, 1951. These materials are characterized by a composition of primarily noncubic crystals of polyoxides of iron and at least one of the metals barium, strontium and lead and, if desired, calcium. Said materials may be a ferromagnetic material having as a component essential for the ferromagnetic properties, single crystals and/or mixed crystals of magnetoplumbite structure of compounds MO-6Fe2O3, where M or MFeiaOzv represents one of the metals Pb, Ba or Sr. The aforesaid properties are considerably more pronounced with these vmaterials than with the known materials. With a remanence of 2,000 Gauss, the coercive force may amount to, for example, 1,400 Oersteds. The electrical resistance is then more than ohm/cm. at frequencies of a few mcs/sec., so that eddy current losses are very low.

The airgap may be arranged such that its operative surfaces are in part bounded by a surface of the magnet. If at least one of the ferromagnetic parts which is parallel to the lines of force substantially touches the magnet, the magnetic circuit has a maximum degree of compactness, so that a minimum of clearance space is left.

Further, the magnet may be constituted by such a at body the smallest dimension of which is smaller than one quarter of the smallest outer dimension of one of the pole surfaces.

In a particularly advantageous embodiment of the invention, the magnet is made of a solid flat disc and one or both ferromagnetic parts substantially touch(es) the outer edge of the magnet, the operative surfaces of the air-gap being formed in part by the outer edge of the magnet. This embodiment has the advantage that the stray field is minimized.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying diagrammatic drawing, given by way of example.

Fig. 1 shows a disc-shaped permanent magnet 1, which is magnetized in the direction of the arrow. The two pole surfaces 2 and 3 are each adjacent a ferromagnetic (soft iron) part 4 and 5 respectively. The disc 1 has a cylindrical aperture 6, in which is introduced a stud 7 of ferromagnetic material, in which the path of the lines of force is parallel to the direction of the lines of force in the magnet and which is secured to the ferromagnetic part 4.

n The ferromagnetic portion 7 is arranged at a distance 8 from the magnet 1, which is smaller than half, preferably smaller than one quarter the length 9 of the magnet, for example, equal to the width of the airgap 10. The airgap l0 is annular and is located at least in the proximity of the edge 11 of the magnet 1 and may even be adjacent this edge. In the present embodiment, the coil 12 secured to a core 13 of an electrodynamic loudspeaker is arranged in the airgap 10. The assembly is very compact, since the stud 7 is at a very small distance 8 from the magnet, so that the clearance space is restricted. It has been found that the two disc-shaped parts 4 and 5 may have a smaller outer diameter than the magnet 1, without a reduction of the field in the airgap. This may be ascribed to the fact that the stray eld occurring in the present case at the outer edge of the magnet is not increased, if the soft iron does not extend to the outer edge of the magnet 1. Thus an economy in soft iron of about 20% may be obtained in the parts 4 and 5. A small decrease of the outer diameter of the disc-Shaped parts 4 and 5 may give rise at rst to an increase in the field strength in the airgap 10 and a further decrease in the outer diameter may give rise to a decrease in the field strength in the airgap 10.

With the construction shown in Fig. 2, the stray field is materially smaller than with that of Fig. l, since the airgap 10 is formed at the outer edge of the magnet 1 and no central aperture 6 is provided and since the straw field (shown in broken lines) occurring at the outer edge 14 of the magnet 1 (Fig. 1) owing to the small length of the magnet 1 which stray field is not used effectively, is with the construction shown in Fig. 2 concentrated for a large part in the airgap 10 due to the presence of the annular lip on the ferromagnetic part 4 and is thus used effectively. In certain cases, the embodiment shown in Fig. 2 may have a limitation in that the annular airgap 10 has a large diameter, since the latter is determined by the diameter of the magnet 1. This limitation does not apply to the embodiment shown in Fig. l.

A modification of the embodiment shown in Fig. 2 is shown in Fig. 3, in which the soft iron plate 5 has a larger diameter than the magnet 1, the airgap still being in the proximity of the magnet. Referring to Fig. 4, the airgap is provided intermediate the plane surfaces of the magnet 1.

Fig. 5 a modification of the embodiment shown in Fig. l, in which the central stud 7 is practically adjacent the magnet 1, without producing appreciably losses and Fig. 6 shows a further modification.

Fig. 7 shows a modification of the embodiment shown in Fig. 2, in which part of the soft iron surrounding the magnet is in contact with the magnet and Figs. 8 and 9 show similar modifications of the embodiments shown in Figs. 3 and 4, in which substantially all the stray field is concentrated in the airgap and in which the construction has the maximum degree of compactness, so that no clearance space is left.

Referring to Figs. 1, 2, 5 and 7, the operative surfaces of the airgap 10 are in part bounded by a surface of the magnet 1, so that again reduction of the stray field outside the airgap is obtained.

Fig. 10 shows a further embodiment in which, in spite of the fact that the soft iron part surrounding the magnet even extends beyond the pole surface 3, no magnetic shortcircuit occurs. In Figs. 6, 8, 9 and 10, the airgap 10 is in part bounded by a surface of the magnet.

Fig. l1 shows a U-shaped magnetic circuit, in which the soft iron part 11 is made in one piece and is adjacent the lower pole surface 2, whilst the airgap is formed between the part 13 and the pole surface 3. Such a system may, for example, be used as a braking magnet for a Ferraris meter.

A modification of this embodiment of Fig. l1 is shown in Fig. 12, in which the airgap is provided between two permanent magnets 14 and 15.

In these cases, the airgap is bordered by an entire pole surface (Fig. 11) and by two pole surfaces (Fig. l2).

The magnetic circuits described above are particularly suitable for use in magnetic filters, measuring instruments and loudspeakers. A simple magnetic filter, for example, an oil filter, may be obtained by combining a number of the co-axial circuits shown in Figs. 8, 9 l1 and 12.

What we claim is:

1. A magnete circuit having at least one air-gap therein comprising a ferromagnetic part having a high permeability and a substantially fiat thin permanent magnet having a smaller dimension in a given direction than dimensions at right angles thereto and opposed flat faces extending at right anges to said given direction. said magnet being magnetized along an axis parallel to said given direction thereby producing magnetic poles on its fiat faces` said magnet consisting essentially of non-cubic crvtals of a material selected from the group consisting of MO-6Fe2O3 and MFeiQOzv, M being at least one of the metals selected from the group consisting of barium, strontium and lead, said material having a coercivity of at least 750 Oersted and a low remanence and a permeability of less than 2, said fiat faces having a given dimension at right angles to said given direction which is larger than any other dimension at right angles to said given direction, said smaller dimension in said given direction being smaller than one-quarter of the largest dimension of the fiat faces, said ferromagnetic part having a portion abutting a face of said magnet and another portion extending parallel to said given direction and spaced from said magnet a distance which is smaller than onehalf of said smaller dimension, said air-gap being located at the end of said other portion of said ferromagnetic part.

2. A magnetic circuit having at least one air-gap therein comprising a ferromagnetic part having a high permeability and a substantially fiat thin permanent magnet having a smaller dimension in a given direction than dimensions at right angles thereto and opposed flat faces extending at right angles to said given direction, said magnet being magnetized along an axis parallel to said given direction thereby producing magnetic poles on its fiat faces, said magnet consisting essentially of non-cubic crystals of a material selected from the group consisting of MO6Fe2O3 and MFeisOzv, M being at least one of the metals selected from the group consisting of barium, strontium, and lead, said material having a coercivity of at least 750 Oersted and a low remanence and a permeability of less than 2, said flat faces having a given dimension at right angles to said given direction which is larger than any other dimension at right angles to said given direction, said smaller dimension in said given direction being smaller than one-quarter of the largest dimension of the fiat faces, said ferromagnetic part embracing said permanent magnet and including portions abutting opposed faces of said magnet and another portion extending parallel to said given direction and spaced from said magnet a distance which is smaller than one-half of said smaller dimension, said air-gap being located at the end of said other portion of said ferromagnetic part.

3. A magnetic circuit having at least one air-gap therein comprising a pair of ferromagnetic parts each having a high permeability and a substantially flat thin permanent magnet having a smaller dimension in a given direction than dimensions at right angles thereto and opposed flat faces extending at right angles to said given direction, said magnet being magnetizcd along an axis parallel to said given direction thereby producing magnetic poles on its fiat faces, said magnet consisting essentially of noncubic crystals of a material selected from the group consisting of MO6Fe2O3 and MFeisOzi, M being at least one of the metals selected from the group consisting of barium, strontium and lead, said material having a coercivity of at least 750 Oersted and a low remanence and a permeability of less than 2, said fiat faces having a given dimension at right angles to said given direction which is larger than any other dimension at right angles to said given direction, said smaller dimension in said given direction being smaller than one-quarter of the largest dimension of the fiat faces, said ferromagnetic parts each comprising a cup-shaped member embracing said permanent magnet and including portions abutting opposed faces of said magnet and another portion extending parallel to said given direction and spaced from said magnet a distance which is smaller than one-half of said smaller dimension, said air-gap being annular and being located at the end of said other portion of said ferromagnetic parts.

4. A magnetic circuit as claimed in claim 3 in which said other portion of said ferromagnetic parts extending parallel to said given direction abuts said permanent magnet.

5. A magnetic circuit having at least one air-gap therein comprising a pair of ferromagnetic parts each having a high permeability and a substantially fiat thin permanent magnet having a smaller dimension in a given direction than dimensions at right angles thereto and opposed flat faces extending at right angles to said given direction, said magnet being magnetized along an axis parallel to said given direction thereby producing magnetic poles on its fiat faces, said magnet consisting essentially of non-cubic crystals of a material selected from the group consisting of MO-6FezOs and MFeiOzi, M being at least one of the metals selected from the group consisting of barium, strontium and lead, said material having a coercivity of at least 750 Oersted and a low remanence and a permeability of less than 2, said fiat faces having a given dimension at right angles to said given direction which is larger than any other dimension at right angles to said given direction, said smaller dimension in said given direction being smaller than onequarter of the largest dimension of the fiat faces, one of said ferromagnetic parts comprising a cupshaped member embracing said permanent magnet and including a portion abutting a face of said magnet and another portion extending parallel to said given direction and spaced from said magnet a distance which is smaller than onehalf of said smaller dimension, said other ferromagnetic member comprising a fiat member abutting the other face of said magnet, said airgap being located at the end of said other portion of said ferromagnetic part.

6. A magnetic circuit as claimed in claim 5 in which said other portion of said cup-shaped member abuts said magnet.

7. A magnetic circuit having at least one air-gap therein comprising a pair of ferromagnetic parts each having a high permeability and a substantially at thin permanent magnet having a central aperture and a smaller dimension in a given direction than dimensions at right angles thereto and opposed flat faces extending at right angles to said given direction, said magnet being magnetized along an axis parallel to said given direction thereby producing magnetic poles on its tlat faces, said magnet consisting essentially of non-cubic crystals of a material selected from the group consisting of MO'FezOs and MFe1sO27, M being at least one of the metals selected from the group consisting of barium, strontium and lead, said material having a coercivity of at least 750 Oersted and a low remanence and a permeability of less than 2, said flat faces having a given dimension at right angles to said given direction which is larger than any other dimension at right angles to said given direction, said smaller dimension in said given directionV being smaller than one-quarter of the largest dimension of the ilat faces, one of said ferromagnetic parts comprising a lat portion abutting a face of said magnet and another projecting portion extending through said central aperture of said magnet parallel to said given direction and spaced from said magnet a distance which is smaller than one-half of said smaller dimension, said other ferromagnetic part comprising a ilat plate including an aperture abutting the other face of said magnet and being spaced from said other projecting portion, said air-gap being located between said projecting portion and said other ferromagnetic part.

8. A magnetic circuit as claimed in claim 7 in which said other projection portion of said one ferromagnetic part abuts said magnet.

References Cited in the le of this patent UNITED STATES PATENTS 2,400,662 Roberton et al. May 21, 1946 FOREIGN PATENTS 406,086 Great Britain Feb. 22, 1934 867,783 France Nov. 27, 1941 

